A mixer is commonly used as a frequency converting component in a wireless transceiver. FIG. 1 shows a conventional wireless transceiver 10. The wireless transceiver 10 comprises filters 11 and 12, programmable gain amplifiers 13 and 14, mixers 15 and 16, and a power amplifier 17. Given a baseband input signal I, for example, the baseband input signal I is filtered by the filter 11 to remove unwanted frequency components, amplified or attenuated using the programmable gain amplifier 13, and transmitted into the mixer 15. The baseband input signal I is then converted to a radio frequency specified by standards according to an oscillation signal LOI generated by a local oscillator (not shown). Finally, the converted signal is amplified by the power amplifier 17 so as to proceed with wireless transmission. In the wireless transceiver 10, frequency conversion performed by the mixers 15 and 16 plays a crucial role in determining signal quality of the wireless transmission.
FIG. 2 shows a circuit diagram of a conventional mixer. A Gilbert mixer 20 comprises a transconductor 21, a switch quad 22 and a load circuit 23. The load circuit 23 includes loads 231 and 232. The loads 231 and 232 each has one end thereof coupled to a voltage supply Vcc, and the other end thereof being an output end. The switch quad 22 comprises NMOS transistors M3, M4, M5 and M6. The drains of M3 and M5 are coupled to one end of the load 231. The drains of M4 and M6 are coupled to one end of the load 232. In addition, the gates of M3 and M6 are coupled to each other, and the gates of M4 and M5 are coupled to each other. The gates of M3 and M4 may receive a local oscillator signal LO. The sources of M3 and M4 are coupled to form a first current path. The sources of M5 and M6 are coupled to form a second current path.
The transconductor 21 comprises NMOS transistors M1 and M2. The drain of M1 is coupled to the first current path of the switch quad 22. The drain of M2 is coupled to the second current path of the switch quad 22. The gates of M1 and M2 receive voltage signals Vin+ and Vin−, respectively. The sources of M1 and M2 are coupled to each other. Between the source of M1 and a ground terminal is an NMOS transistor MS. The gate of transistor MS is inputted with a constant voltage to facilitate the transistor MS in forming a current source.
FIG. 3 shows a schematic diagram of signals associated with the mixer 20. The transconductor 21 converts a differential input voltage signal Vin, that is, Vin+ or Vin−, to a current signal Ib. When passing through the first current path and the second current path of the switch quad 22, the current signal Ib is driven by the oscillator signal LO to generate a frequency-converted current signal. The frequency-converted current signal is next converted by the load circuit 23 to generate an output voltage at the output end.
The transconductor 21 consists of the NMOS transistors M1 and M2, and therefore a voltage-current relationship of the transconductor 21 is a conic relationship rather than a linear relationship. To be more specific, the prior art mixer shown in FIG. 2 is unsuitable for applications on mixers that are in need of high linearity, such as in Wireless Local Access Network (WLAN) transmitters and Code Division Multiple Access (CDMA) system transmitters.